chapter 10 acids and bases. review chapter 6 defined an arrhenius acid and base defined a...
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The Brønsted-Lowry Concept of Acids and Bases Brønsted-Lowry definitions
Acids are proton (H+) donors.
The Brønsted-Lowry Definition of Acids and Bases Putting the acid and base definitions together, an acid–base
reaction is one in which a proton is transferred. The reaction need not occur in water.
Conjugate Acid-Base Pairs Result from the transfer of a proton The acid reactant loses the proton to produce a conjugate
base product The base reactant gains the proton to produce a conjugate
acid product Hence a conjugate acid-base pair are two chemicals (one
reactant and one product) that differ by a hydrogen ion
Base Acid+Acid Base+
Conjugate Pair
Conjugate Pair
Reaction 4 H2PO4- OH-+
Reaction 5 H2SO4 N2H5++
Reaction 6 HPO42- SO3
2-+
Reaction 1 HF H2O+ F- H3O++
Reaction 3 NH4+ CO3
2-+
Reaction 2 HCOOH CN-+ HCOO- HCN+
NH3 HCO3-+
HPO42- H2O+
HSO4- N2H6
2++
PO43- HSO3
-+
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Acid and Base Strength Also introduced in Chapter 6 and discussed Chapter
7 and 9 – Acid/Base strength The strength of an acid/base, described by the
equilibrium constant, describes the degree to which the compound dissociates into its ions Strong acids and bases dissociate entirely in water – large
equilibrium constants Weak acids only partially ionize and therefore an
equilibrium is established – small equilibrium constants
Strong acid: HA(g or l) + H2O(l) H3O+(aq) + A-(aq)
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Acid and Base Strength
Weak acid: HA(aq) + H2O(l) H3O+(aq) + A-(aq)
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Acid and Base Strength
Acid Dissociation Contants Ka – indicates the degree to which an acid will
ionize Stronger acid – Ka >> 1 meaning complete
dissociation Weaker acid – Ka < 1
hence reactant favored
Acids
Strong
hydrochloric acid, HCl
hydrobromic acid, HBr
hydroiodic acid, HI
nitric acid, HNO3
sulfuric acid, H2SO4
perchloric acid, HClO4
Weak
Begin with H or contains a carboxylic acid group, -COOH
Bases
Strong
Weak
sodium hydroxide, NaOH
calcium hydroxide, Ca(OH)2
potassium hydroxide, KOH
strontium hydroxide, Sr(OH)2
barium hydroxide, Ba(OH)2
lithium hydroxide, LiOH
Amine group - NHx
Acid and Base Strength
Acid and Base Strength An acid base reaction always moves toward
the weaker acid and the weaker base
Conjugate pairs the weaker the acid, the stronger its conjugate
base
the weaker the base, the stronger its conjugate base
Acid Dissociation Constants Monoprotic acid – acids that have but one
proton to give up – HCl, HF, HNO3, CH3COOH
Polyprotic acids – acids that can give up more than one proton – H2SO4, H3PO4, oxalic acid Ionization occurs in a step wise manner
Polyprotic acids
acids with more than more ionizable proton
H3PO4(aq) + H2O(l) H2PO4-(aq) + H3O+(aq)
H2PO4-(aq) + H2O(l) HPO4
2-(aq) + H3O+(aq)
HPO42-(aq) + H2O(l) PO4
3-(aq) + H3O+(aq)
Ka1 =[H3O+][H2PO4
-]
[H3PO4]
Ka2 =[H3O+][HPO4
2-]
[H2PO4-]
Ka3 =[H3O+][PO4
3-]
[HPO42-]
Ka1 > Ka2 > Ka3
= 7.2x10-3
= 6.3x10-8
= 4.2x10-13
Acid Dissociation Constants
AC
ID S
TR
EN
GT
H
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Water as Both an Acid and a Base Acid reaction with water
HA (aq) + H2O (l) → H3O+ (aq) + A- (aq) H2O acts as a Brønsted-Lowry base
Base reaction with water B: (aq) + H2O (l) → BH+ (aq) + OH- (aq) H2O acts as a Brønsted-Lowry acid
Water is amphoteric An amphoteric species is a substance that can react as an acid or a
base
The Auto-ionization of Water Highly purified water
auto-ionization of water Conduct a small amount of electricity
H2O(l) H2O(l)
H3O+(aq) OH-(aq)
+
[H3O+][OH-]
Ion Product Constant for Water
Kw =
A change in [H3O+] causes an inverse change in [OH-].
= 1.0 x 10-14 at 250C
H2O(l) + H2O(l) H3O+(aq) + OH-(aq)
In an acidic solution, [H3O+] > [OH-]
In a basic solution, [H3O+] < [OH-]
In a neutral solution, [H3O+] = [OH-]
[H3O+] [OH-]Divide into Kw
ACIDIC SOLUTION
BASIC SOLUTION
[H3O+] > [OH-] [H3O+] = [OH-] [H3O+] < [OH-]
NEUTRAL SOLUTION
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Measuring Acidity in Aqueous Solution: pH pH scale allows the
expression of H3O+ concentration in less bulky numbers
pH is a number usually between 0 and 14 that indicates the hydronium concentration
Mathematically determined
pH = -log [H3O+]
10 -pH = [H3O+]
Buffer Solutions Buffer: A combination of substances that act
together to prevent a drastic change in pH; usually a weak acid and its conjugate base.
Rearranging the Ka equation shows that the value of [H3O+] depends on the ratio [HA]/[A-].
[H3O+] = Ka [HA]/[A-] Most H3O+ added is removed by reaction with A- ,so
[HA] increases and [A-] decreases. As long as these changes are small, the ratio [HA]/[A-] changes only slightly, and there is little change in the pH.
When 0.010 mol of acid and 0.010 mol of base are added to 1.0 L of pure water and to 1.0 L of a 0.10 M acetic acid–0.10 M acetate ion buffer, the pH of the water varies between 12 and 2, while the pH of the buffer varies only between 4.85 and 4.68.